That's an interesting problem. On the face of it, you have done what I would have done. My next logical step is to use AI as it's a good tool sometimes. Below are some steps AI recommends to check. Hope you can figure it out!
1. Rule Out the Bulbs (Even If Unlikely)
Both low beams failing at once while driving is rare for bulbs, but it's possible if they were aging or there's a shared circuit issue causing overload. Your 2017 Civic likely uses H11 bulbs for lows (confirm your trim: LX/EX use halogen; higher trims may have LED).
Next step: Replace both low beam bulbs with new ones (cheap and quickaround $20-30 for a pair). Or, test by swapping in known-good bulbs from another car. If highs work, the sockets/connections aren't completely fried.
Why? In some cases, one bulb failing can strain the circuit and take out the other shortly after.
2. Check Voltage at the Low Beam Connectors
This will tell you if power is reaching the bulbs when lows are selected. No voltage points to the relay, switch, wiring, or module; voltage present means bad bulbs, grounds, or connectors.
How to do it:
Park safely, engine off, key in ON position (or running if needed for lights).
Access the headlights: Pop the hood, remove the dust caps behind each headlight assembly, and unplug the low beam connector (it's a 2-pin plugpower and ground).
Set your multimeter to DC volts (20V range).
Turn the headlight switch to low beams.
Probe the two pins in the connector: You should see ~12V DC across them. (Polarity: Ground is usually the black wire; power might be red/blue, red/yellow, or white depending on sideleft vs. right. If unsure, test both ways; voltage will show positive or negative but should be there.)
Test both sides.
If no voltage: Proceed to relay/switch checks below.
If voltage present but lights don't work: Clean the connector pins (corrosion is common), check the ground wire continuity (multimeter on ohms, probe ground pin to chassis metalshould be near 0 ohms), or replace bulbs if not already done.
Tip: If you get flickering or low voltage, it could be a bad ground shared by both lightstrace the black ground wires back to their chassis bolt and clean/tighten.
3. Re-Test the Low Beam Relay (Even If You Think It's Good)
You mentioned relays check out, but if you only visually inspected or tested continuity, it might still be faulty internally (e.g., stuck contacts). 2017 Civics have a single low beam relay controlling both sides, while highs are separate.
Location: In the under-hood fuse box (driver's side engine bay). Remove the coverthe relay is typically labeled "HEADLIGHT LOW" or similar (position R4 or nearby; it's a small square plug-in module). Refer to your owner's manual or the fuse box lid diagram for exact spot.
Next step: Swap it with an identical relay from the box (e.g., high beam relay if matching part number, or horn/fan relaythey're often interchangeable Omron types like 39794-SDA-A03). Turn on lows and test. If it works, buy a replacement relay (~$10-20 at auto parts store or Honda dealer).
Why common? Relays fail from heat/vibration, and forums report this fixing dual low beam outages. Videos show this exact check for your year.
4. Test the Headlight Switch (Since You Already Pulled It)
The switch (combination lever on the steering column) could have failed in the low beam position, especially if there's wear or a bad contact.
For continuity test: You need the pinout. The switch has a multi-pin connector (usually 12-14 pins). Common low beam output pins are:
Pin 1 or 2: Battery power in (red wire, ~12V with key ON).
Pin for low beam signal: Often green/red or similarcheck for continuity between input power and this output pin when switch is in low position.
Ground: Black wire.
Exact pinout varies by trim, but you can find it in service manuals (more below). Use multimeter on ohms: With switch disconnected, set to low beams, probe pins to confirm the circuit closes (0 ohms) for lows but not highs.
If no continuity, replace the switch assembly (~$100-200 part).
Note: The switch signals the MICU (under-dash control unit), which then activates the relayso a MICU glitch could mimic this. The initial dash trouble lights hint at a module fault (e.g., temporary short), but that's rarer.
5. If Still Stuck: Wiring Diagram and Further Help
Free diagrams are spotty online, but check civicx.com forums (search your symptomslots of 2017 threads) or honda-tech.com. For a full service manual with wiring (including color codes and pinouts), buy/download one from Helm Inc. (official Honda, ~$100) or search for "2017 Honda Civic electrical troubleshooting manual PDF" on sites like manualsLib or eBay.
Example wire colors from similar models: Left low power red/yellow, right red/blue, grounds black. But verify with manual to avoid shorts.
If voltage checks show no power and relay/switch are good, scan for codes with an OBD2 tool (cheap ones work; look for lighting module errors). Could be wiring harness damage (rodents, corrosion) or MICU failuredealer territory then, as MICU replacement is ~$300-500 + programming.
Start with bulbs and voltagethat'll narrow it down fast.
